Photovoltaic devices recently have attracted attention due to energy shortage on Earth. The photovoltaic devices can be boldly classified into crystalline silicon solar cells and thin film solar cells. Crystalline silicon solar cells are the main stream photovoltaic device owing to its mature manufacturing technology and high efficiency. However, crystalline silicon solar cells are still far from common practice because its high material and manufacturing cost. Thin film solar cells are made by forming a light absorbing layer on a non-silicon substrate, such as glass substrate. Glass substrate has no shortage concern and the price thereof is cheaper as comparing with silicon wafers used in crystalline silicon solar cells. Therefore, thin film solar cells are considered as an alternative to crystalline silicon solar cells.
Thin film solar cells can be further classified by material of the light absorbing layers, such as amorphous silicon, multi-crystalline silicon, Cadmium Telluride (CdTe), Copper indium gallium selenide (CIS or CIGS), Dye-sensitized film (DSC) and other organic films. Among these thin film solar cells, CIGS solar cell has reached cell efficiency of 20%, which is comparable with crystalline silicon solar cells.
The quaternary semiconductor Cu2ZnSn(S,Se)4 (CZTS), having a crystalline structure similar to CIGS, is a new photovoltaic material which attracts interests recently due to its low cost natural abundant and non-toxic elements. Conventional methods for forming CZTS films are processed under vacuum environment. It is reported that Ito and Nakazawa prepared CZTS thin films on a stainless steel substrate by atom beam sputtering. Friedl Meier et al. prepared CZTS thin films by thermal evaporation and the CZTS solar cells prepared by this method had a conversion efficiency of 2.3%. Katagiri et al. prepared CZTS thin films by RF sources co-sputtering followed by vapor phase sulfurization or by sulfurizing electron-beam-evaporated precursors and the efficiency of the resulted CZTS solar cell was 6.77%.
As described above, conventional methods for forming the CZTS solar cells usually utilize vacuum processes. However, vacuum processes are in general quite expensive and the cost of the CZTS solar cells is thus increased. Therefore, a solution process which does not require vacuum equipment is desired in order to reduce the manufacturing cost.